1. Field of the Invention
The invention is generally related to the area of optical communications. In particular, the invention is related to dispersionless multimode optical tap filters.
2. The Background of Related Art
To meet the ever-increasing demands for high bandwidth and more flexibility in modern communication networks, utilizing optical fiber networks capable of transmitting multiplexed channel signals is becoming increasingly popular. Many optical devices have been designed to meet the demands. Practically, these demands are arising from three areas: 1) video traffic over the Internet, such as Netflix and Youtube etc., 2) mobile access via smart phones including voice over IP (VoIP), 3) cloud storage and rich media files migrating via the Internet. All these demands seem to boost considerably the requirements on the service provider and the booming of various data centers. To meet the higher speed requirement, service providers and data centers also need more flexible network devices or equipment to support debugging and monitoring of the signals originated from them.
The availability of vertical cavity surface emitting laser (VCSEL) technology facilitates high speed parallel data links that can transmit 12 to 24 parallel channels, each channel supporting more than 10 gigabit/sec (Gbps). VCSEL has recently been demonstrated for bandwidth even higher than 25 Gbps and promises to provide low cost parallel data links among computers, storage devices and servers to form an aggregated data center. As networked computers and storage devices grow larger and larger in functions and complexities, monitoring the network becomes an essential task to ensure the communication quality and data integrity.
In the past, a data center used a conventional fuse coupler for tap monitoring function. An optical tap is essentially a 1×2 device, where a portion of the input signal is branched out to a photo-detector that converts the tapped signal to serve as a monitored signal. FIG. 1 shows one of the simplest optical links, a 1×2 coupler enables traffic access port (tap) to monitor the network.
By employing signals at two different wavelengths respectively, bi-directional signals can be conveniently carried on the duplex fibers as shown in FIG. 2 Two wavelengths λ1 and λ2 travel in the two opposite directions, respectively, and the 2×2 coupler allows tap monitoring for both channels.
A typical coupler splits the power of the input signal to different ports in a desired power ratio. It is originally a 3-port device where a portion of the input signal is branched out to a photo-detector that converts the tapped signal to a monitored signal. Depending on the applications, the power splitting ratio (tap-ratio) for the two wavelengths λ1 and λ2 should be either the same or different for the two wavelengths λ1 and λ2. For example, a bi-directional link may have λ1=850 nm and λ2=910 nm with same tap-ratio as 70% power on signal and 30% on tap, or different tap-ratios for the two channels as 7:3 for 850 nm but 8:2 for 910 nm.
FIG. 3 duplicates FIG. 2 of US Pat. Pub. No.: 2005/0201677 to show the structure of an exemplary multimode optical fiber coupler. Although the fiber can be either single mode or multimode for a fused coupler tap filter, the rapid growth of vertical cavity surface emitting laser (VCSEL) technology, which allows high speed parallel data links that can transmit 12 to 24 parallel channels, makes the multimode tap filter more important because the multimode oxide VCSEL offers modulation and reliability improvement comparing to that of single mode and has become the promising technology for high speed communication. However, the multimode fused optical coupler shows its fundamental limitation in high speed networks. As the data rate goes higher, the multimode fused optical coupler would have mode oriented insertion loss degradation, especially for coupler with non-symmetry tapping/splitting ratio. The degradation could be several dB in bit-error-rate (BER) testing. This is known as the modal dispersion due to the different group velocities for the higher and lower order modes propagating in multimode fiber. Since the dispersion also depends on the tap ratio, the outputs of express ports of a non-50/50 fused coupler have different dispersion. It limits the data bandwidth in a communication link especially in high speed (e.g., 10 Gbps and more).
The main cause for mode dispersion can be explained by the coupled-mode theory (“Simplified theory of the multimode fiber coupler”, The Bell System Technical Journal, Vol. 56 No. 5, Page 729). In addition, the mode coupling in waveguide results in mode dependent loss. These effects are the fundamental limitation of fused optical coupler and therefore new design based on thin film filter was developed to fulfill the high speed application.
Fused fiber based coupler may have another difficulty in the 2×2 configuration for bi-directional link because the tap-ratio for each wavelength is not fully controllable independently. When the two wavelengths are relatively far away (e.g., about tens of nm), the wavelength dependent coupling results in uneven tap-ratios for the two channels. And it is extremely difficult to tune the tap-ratios for the two wavelength channels to the desire (7:3 and 8:2 for each, for instance) separately in fused coupler.
The present invention disclosure teaches a solution for monitoring optical power, dispersion and mode distribution in high speed multimode fiber communication. One embodiment of the present invention is a multimode optical tap filter that is dispersionless and tap ratio independent.